Structure elements comprising “inflatables” are known in the art. See, for example, the AirBeams™of Vertigo, Inc. at www.vertigo-inc.com. One such element is an arch that is made of a woven fabric exterior and an internal membrane that is pressurized with air. The arch further comprises “cohesionless” particles that are compressed against the fabric exterior by air pressure inflating the internal membrane. This “hydrostatically enabled” arch, when stabilized by suitable guy wires, is able to support an SUV hanging from its center, much more than otherwise possible without the addition of the particles. Tension straps on the top and bottom are used for additional reinforcement to support the heavy loads.
This demonstration of the concept has led to plans for further development by the U.S. Army, specifically the Inverse Triaxial Structural Element (ITSE) Project with a goal of developing a practical demonstration of the use of very high performance tensile fabrics. The approach is to develop and test the concept using existing fabrics, using structural test results to calibrate and validate and develop a finite element model (FEM) of structure. A validated FEM model would then be used with a continuum model to predict enhancement of fabric materials, in particular those employing carbon nanotubes (CNT), and structure using the CNT fabric.
In support of the ITSE Project, the Army developed a test structure for testing the basic concept of “hydrostatic enablement.” The concept of the test structure is illustrated in FIG. 1. Refer to FIG. 1, showing a top view of a test apparatus 10 with the center section 12 further depicted for illustration purposes only. A test device 10 incorporating a reinforced rigid external cylinder 11 incorporates a center 12 comprising a flexible tube filled with cohesion-less particles 14, such as dry sand, the cylinder 11 filled with water 15. The water 15 is pressurized to a pressure represented as σ3 to enable the center column to withstand a load represented as σ1. As the value of σ3 increases to a pre-specified amount the available loading capacity of σ1 also increases to a pre-specified amount as the center column of particles 14 stiffens under the increasing compressive force σ3. This is best seen in FIG. 1B in which a first “differential” stress-strain curve 17 depicts the relationship between σ3 and σ1 for a “nominal value” of σ3. As σ3 is increased by increasing the water pressure in the cylinder 10, the value of σ1 also increases as indicated by the differential stress-strain curve 16 and the dashed curve 18 indicating the significant increase in slope of the differential curve 16 with an increase in σ3. This follows the Mohr-Coulomb relation for cohesion-less soils:τ=(σ−μ)tan(φ)+c   (1)where:
τ=shear strength (stress)
σ=normal stress
c=cohesion (intercept of failure envelope with τ axis)
φ=slope of the failure envelope (angle of internal friction)
μ=hydrostatic pressure
The U.S. Army has investigated using thin wall structures for “hydrostatically enabled” structure elements. Refer to FIG. 2. In FIG. 2A, a “support column” 202 of cohesion-less particles 203, such as dry sand, encased in a flexible membrane 204, such as butyl rubber or the like, is compressed and made more rigid by the use of pressure, σc′, equally impressed over its length. FIG. 2B is a top view of the thin-walled tube 202 showing the opposing force, σc′, inside the thin-walled tube, the relationship to tensile force, T, given by:σc′=Td/2t   (2)where:
T=tensile force in a thin-walled cylinder
d=diameter of a thin-walled cylinder
t=thickness of the thin wall
σc′=hydrostatic pressure applied
Eqn. (2) may be used to design appropriately sized systems based on the basic theory of the Mohr-Coulomb relation of Eqn. (1) and pre-specified loads, σ, expected. For example, a designer can specify the thickness, t, and diameter, d, of a thin-wall tube based on how much hydrostatic pressure will need to be applied to support a pre-specified axial load, σ.
An alternative depiction of the effect of “stiffening” of cohesion-less particles is shown in FIG. 2C, a stress-strain curve, indicating how a low applied hydrostatic pressure, σcL′, exhibits a significantly lower load, σ1′, than a higher applied hydrostatic pressure, σcH′, at the same slope of the failure envelope, φ′.
Refer to FIG. 3A, a test configuration 301 for the ITSE. The filled tube 301 comprises an outer membrane 302 of abrasion resistant material, such as woven Kevlar® or the like, an inner bladder 304 of flexible material, such as urethane, butyl rubber or the like, and a “fill” of cohesion-less particles 305, such as dry sand of medium density. A suitable fluid 303, such as air, is employed to inflate the inner bladder 304 and provide the necessary pressure to stiffen the particles 305 into a rigid mass impressed against both the bladder 304 and the outer membrane 302. FIG. 3B is a loading layout of the configuration 301 of FIG. 3A, the configuration 301 emplaced upon supports 306, prior to impressing a load, σ2. Testing demonstrated the viability of the ITSE concept. The filled tubes for the test were about 10.2 cm (four inches) in diameter and about 61 cm (two feet) in length. They had a compliant internal urethane bladder and an external membrane of polyester bias braid, the same material as the air arch that supported an SUV. The internal bladder was inflated to 100 psi, providing axial loading to full mobilization of the shear strength of the particulates, dry sand, or of either membrane. A 3-point bending test was conducted to full mobilization of the shear strength of the soil or of either the internal bladder or external membrane.
Test results are shown in the graphs of FIGS. 4 and 5. FIG. 4 shows results for two test units in compression, showing less than about 3.8 cm (1.5 in.) extension for a load in excess of 4,000 lbs and less than about 4.4 cm (1.75 in.) extension for a load of about 5,400 lbs, making the unit able to carry a load about 12 times greater than a tube filled only with dry sand. FIG. 5 shows a linear deflection curve of flexural force (psi) vs. deflection (in.), topping near 1000 psi at a deflection of only about 5.1 cm (two inches).
U.S. Pat. No. 6,463,699, Air Beam Construction Using Differential Pressure Chambers, to Bailey, describes a closed tubular cylindrical shell of air impermeable fabric having fixed within the shell an “I-beam envelope” comprising flexible, air impermeable walls sealed to the interior of the shell. The I-beam envelope extends the length of the shell and defines air chambers in communication with an inflation valve. Compressible material is dispersed throughout the interior of the I-beam envelope. When subjected to compressive forces by pressurization of the air chambers the material becomes rigid, thus able to support increased loading, albeit horizontal in the normal orientation of I-beams. The filled envelope is either vented to atmosphere or connected to a vacuum source.
The above demonstrates the feasibility of hydrostatically enabled structure elements but does not address many of the practical considerations for use of the technology. One such consideration is use of these structure elements in addressing damages to existing structure to mitigate further catastrophic deterioration, injury or loss of life. Select embodiments of the present invention address this and other practical applications.